Aphids are difficult to control once their populations get large. You can repeatedly spray them off your plants with a strong stream of water, or you can resort to using Insecticidal Soap. It's made of fatty acids from certain plants and is effective against soft-bodied insects like aphids. Aphids are especially difficult to control because they bear live young that are capable of bearing live young almost immediately. Therefore, you can have three or more generations feeding on the same leaf and stem at the same time! You can use Insecticial Soaps every 7-10 days for control. Read and follow label directions.